LTC3561A datasheet(14/20 Pages) LINER

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LTC3561A

3561af

Checking Transient Response

The OPTI-LOOP

® compensation allows the transient re-

sponse to be optimized for a wide range of loads and output

capacitors. The availability of the ITH pin not only allows

optimization of the control loop behavior but also provides

a DC coupled and AC ﬁltered closed loop response test

point. The DC step, rise time and settling time at this test

point truly reﬂects the closed loop response. Assuming a

predominantly second order system, phase margin and/or

damping factor can be estimated using the percentage of

overshoot seen at this pin. The bandwidth can also be

estimated by examining the rise time at the pin.

The ITH external components shown in the circuit on page 1

of this data sheet will provide an adequate starting point for

most applications. The series R-C ﬁlter sets the dominant

pole-zero loop compensation. The values can be modiﬁed

slightly (from 0.5 to 2 times their suggested values) to

optimize transient response once the ﬁnal PC layout is

done and the particular output capacitor type and value

have been determined. The output capacitors need to be

selected because the various types and values determine

the loop feedback factor gain and phase. An output current

pulse of 20% to 100% of full load current having a rise

time of 1μs to 10μs will produce output voltage and ITH

pin waveforms that will give a sense of the overall loop

stability without breaking the feedback loop.

Switching regulators take several cycles to respond to a

step in load current. When a load step occurs, VOUT im-

mediately shifts by an amount equal to

ΔILOAD•ESR,where

ESR is the effective series resistance of COUT. ΔILOAD also

begins to charge or discharge COUT generating a feedback

error signal used by the regulator to return VOUT to its

steady-state value. During this recovery time, VOUT can

be monitored for overshoot or ringing that would indicate

a stability problem.

The initial output voltage step may not be within the

bandwidth of the feedback loop, so the standard second

order overshoot/DC ratio cannot be used to determine

phase margin. The gain of the loop increases with R and

the bandwidth of the loop increases with decreasing C.

If R is increased by the same factor that C is decreased,

the zero frequency will be kept the same, thereby keeping

the phase the same in the most critical frequency range

of the feedback loop. In addition, a feedforward capacitor

CF can be added to improve the high frequency response,

as shown in Figure 4. Capacitor CF provides phase lead by

creating a high frequency zero with R2 which improves

the phase margin.

The output voltage settling behavior is related to the stability

of the closed-loop system and will demonstrate the actual

overall supply performance. For a detailed explanation of

optimizing the compensation components, including a

review of control loop theory, refer to Linear Technology

Application Note 76.

Although a buck regulator is capable of providing the full

output current in dropout, it should be noted that as the

input voltage VINdropstowardVOUT,theloadstepcapability

APPLICATIONS INFORMATION

Figure 4. LTC3561A General Schematic

OPTI-LOOP is a registered trademark of Linear Technology Corporation.

PVIN

LTC3561A

SVIN

VFB

ITH

SHDN/RT

OPTIONAL

VIN

SGND PGND

3561A F04

CITH

VOUT

CIN

PGND

SGND

SGND SGND

GND

PGND

COUT

SGND

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